Metal-air cells usually contain as electrochemically active components a metal-based anode and an air cathode separated from one another by an ion-conductive electrolyte. When discharging, oxygen is reduced on the air cathode while taking up electrons. Hydroxide ions which can migrate via the electrolyte to the anode are produced. There, a metal is oxidized by electron donation. The metal ions produced react with the hydroxide ions.
Both primary and secondary metal-air cells exist. A secondary metal-air cell is recharged in that a voltage is applied between the anode and the cathode and the electrochemical reaction described is reversed. Oxygen is released in the process.
The best known example of a metal-air cell is the zinc-air cell. It is used in the form of a button cell in particular as a battery for hearing aids.
Metal-air cells have a very high energy density, since the requirement for oxygen on the cathode can be covered by ambient atmospheric oxygen. Accordingly, atmospheric oxygen has to be supplied to the cathode during the discharging operation. Conversely, oxygen produced on the air cathode during the charging operation of a metal-air cell has to be evacuated. For these reasons, metal-air cells typically have housings provided with corresponding entry and/or exit openings. Typically, holes are stamped into the housings as entry and/or exit openings.
Gas-diffusion electrodes are usually employed in metal-air cells as an air cathode. Gas-diffusion electrodes are electrodes in which the materials involved in the electrochemical reaction (typically a catalyst, an electrolyte and atmospheric oxygen) are present alongside one another in a solid, liquid and gaseous state and can come into contact with one another. When discharging, the catalyst catalyses the reduction of atmospheric oxygen and, if applicable, also the oxidation of hydroxide ions when charging the cells.
Very often, plastic-bound gas-diffusion electrodes are employed as air cathodes in metal-air cells. Such gas-diffusion electrodes are disclosed in DE 37 22 019 A1, for example. In such electrodes, a plastic binder (for example, polytetrafluorethylene (PTFE)) configures a porous matrix in which the particles of an electro-catalytically active material (for example, a precious metal such as platinum or palladium, or of a manganese oxide) are embedded. Those particles have to be able to catalyze the reaction of atmospheric oxygen. Manufacturing of such electrodes most typically takes place by rolling a dry mixture of binder and catalyst to form a film. In turn, the film can be rolled into a metal mesh, for example, a mesh of silver, nickel or silver-plated nickel. The metal mesh forms a conductor structure within the electrode and serves as a current conductor.
The entry and/or exit openings for oxygen are most typically incorporated into the base of the housing of a metal-air cell, in particular, in the case of a button cell. For oxygen entering through the openings to be able to contact the air cathode as immediately as possible, the air cathode in such cells is usually positioned flat on the housing base to cover the openings. If applicable, it may be advantageous to provide an air distributor, for example, a porous filter paper between the air cathode and the housing base. However, this is not required in all cases.
If oxygen is now reduced in an air cathode positioned in this manner, the electrons released are typically evacuated via the conductor structure. The latter is usually directly connected to a part of the housing which can serve as a terminal.
We found in tests that metal-air cells having the narrated construction often have high variance in as far as their electrochemical properties are concerned. In this manner, individual cells typically have markedly higher impedance values than would have been expected on average.